Digital memory circuits are significant components of virtually all digital processing systems, acting as storage of both data values and program instructions. With the increased use of solid-state multimedia devices, the demand for large capacity static read-write digital storage has also experienced a substantial increase. In order to fill this capacity, standard semiconductor memory architectures have been refined using advanced processing techniques in order to increase memory density by shrinking the area requirements of unit memory cells. Additionally, these processing techniques have been used to increase the speed of the static memory architectures. These non-volatile systems generally consist of arrays of memory cells based on semiconductor electronic devices, such as the floating transistors devices in erasable programmable read only memory (EPROM), electronically erasable programmable read only memory (EEPROM), and Flash memory systems. These device architectures currently serve as the basis for such industry standards as Compact Flash and Memory Stick digital storage technologies.
For more advanced applications, such as space-based systems, additional demands are placed on the digital memory systems over increased capacity and non-volatility. These demands include smaller active power consumption per memory unit access, and an increased resistance to radiation effects. The semiconductor electrical systems that are generally used for multimedia applications are extremely susceptible to corruption via radiation-induced errors. This is because the thin oxide layers surrounding the floating gates are susceptible to ionization damage resulting from high energy radiation particles. As a result, heavy radiation shielding is required to protect these memories in hostile environments, which can in turn lead to added costs and thermal management difficulties.
One type of memory architecture which has displayed excellent radiation resistance is magneto-resistive memory. In these types of memory systems, the memory unit cells are based on magnetic components that can be placed into static electrically resistive states. Because the data storage characteristics of these memory cells are based on inherent magnetic as opposed to electrical properties, they are generally immune to ionizing radiation effects. Additionally, they allow for nearly unlimited transitions before the material properties of the memory cells degrade to the point of instability.
This type of memory is based on the principles of the giant magneto-resistive (GMR) effect, which shows that a magnetic field can be used to align the spin of electrons in a ferromagnetic material and thereby change its electrical resistance. In magneto-resistive memory, data is stored in stacks containing thin layers of alternating magnetic and non-magnetic layers. One or more of the magnetic layers may be a bounded layer where the magnetic dipoles of the layers are aligned to a single general reference direction. At least one other magnetic layer in the system may act as a “free layer” whose magnetic dipoles can generally be set to one of two opposite directions, being parallel and anti-parallel to the magnetic dipoles of the bounded layer. Because of the GMR effect, when these stacks are subject to a relatively strong external magnetic field, the spin of the electrons in some of the magnetic layers may be altered in the sense above to be either all aligned or anti-aligned. Based on the formation of the other stack layers, these two states may correspond to distinct high and low resistive states, resulting in either an “open” or “closed” electrical current conducting condition. Specifically, when the spin of the electrons in the free layer is in opposition to that of the bounded layers, the spin-dependent scattering of charge carriers is maximized in the material, thereby creating higher resistance and a “closed” valve. Likewise, when the spin of the electrons in the free layer parallels that of the bounded layer this scattering is reduced, thereby producing an “open” valve. Because of this transitional behavior, the material stacks, or “spin-valves” may be used to store binary data.
In order to read the data stored by the spin-valves, a combination of current drivers and sensing devices may be used to test the resistance of the valve stack layers. When a standard current generated by a current driver is applied to the spin-valve, a voltage potential is generated, which is indicative of either a relatively high or low resistive electrical element. This potential can be sensed by a voltage sense amplifier that may then be used to extract the original state of the valve. Additionally, more advanced components may be included with the voltage sense amplifier in order to increase the accuracy and speed of the sensing sub-system. For example, auto-zeroing sense amplifiers, or chopper-stabilized amplifiers, may be utilized which help to increase the sensitivity of the detection circuitry. An auto-zero sense amplifier having offset calculation is able to account for a continuous internal offset voltage by storing the offsets in one or more coupling capacitors, thereby helping to reduce the overall noise of the detection system.
To change the state of a spin-valve, and thereby write a binary value to the structure, a magnetically-coupled current-carrying write line may be used. Moving charges passing through this write line generate a magnetic field that varies radially in strength, and is directed tangentially to both the axis defined by the direction of current flow in the write line and the radial axis of the write line. In this sense, currents flowing in opposite directions to each other in the same write line generate magnetic fields that are also opposite in direction at the same point in space. The free magnetic layers in a spin valve device will then align to a general direction defined by an applied magnetic field, with the binary directional limitations provided by the bounded layers.
A substantial amount of the overall power utilized by an MRAM system is consumed during writing operations of the individual cells. As a result, it would be desirable to develop a giant magneto-resistive memory cell that has substantially lower per-write power consumption as a result of, for example, a decreased writing current. Additionally, a system that incorporates such a memory cell system should also contain a high-speed sensing subsystem comprising sense amplifiers with the ability to reliably detect low voltage signal levels, thereby permitting the use of lower system voltages and currents that can also provide substantial power savings.